300E fluctuating idle - PeachParts Mercedes-Benz Forum

mike690003 · #1 10-11-2003, 10:08 PM

Hello Board,

My 300E has a wierd idle problem, this happens in "P", "N", or "D". It idles at a steady 500rpm, and then will randomly jump to 700rpm . I have new vacuum lines and vacuum rubber and cleaned the idle control valve. I have new idle control valve hoses also. Does anyone know what could cause this wierd idle problem??

suginami · #2 10-11-2003, 10:17 PM

There are other places where you can have a vacum leak.

We have a different engine with a different fuel injection system, but I had a fluctuating idle, and after replacing a lot of parts, the problem turned out to be a leak at the intake manifold gasket.

Two independent mechanics that specialize in Mercedes failed to find the leak, but the dealer found it in less than an hour.

The moral of the story is:

If you throw enough money at a problem, you are eventually going to fix it.

#3 10-11-2003, 10:34 PM

Do you have a vacuum gauge/pump that you can hook to it while it's idling? If not they're cheap and can be bought at most car parts places. I've used mine dozens of time from checking vacuum operated solenoids to sucking old brake fluid out of the resevoir before adding new. Very handy tool to have.

mike690003 · #4 10-11-2003, 10:53 PM

Is the intake manifold gasket the one located on the airbox??
If so, I have replaced that circular rubber gasket.

I have replaced every visible vacuum line and rubber in the engine bay. Where else could a vacuum leak be coming from??

drbrandini · #5 10-11-2003, 11:09 PM

Mike,

I noticed your other posts regarding the coolant temp. sensor. If its not working correctly or unplugged it will make your car hesitate and run rough. Good Luck

mike690003 · #6 10-11-2003, 11:36 PM

Is that the coolant sensor that controls the aux fan or is it the one that controls the CIS-EZL??

suginami · #7 10-12-2003, 12:24 AM

Quote:

Originally posted by mike690003
Is the intake manifold gasket the one located on the airbox??
If so, I have replaced that circular rubber gasket.

I have replaced every visible vacuum line and rubber in the engine bay. Where else could a vacuum leak be coming from??

No, the intake manifold is made of metal, sits on top of the head, and the gasket is not rubber.

In a combustion engine, a carburetor or fuel injection sits on top of the intake manifold, which directs fuel / air through the intake manifold and into the combustion chambers when the intake valve opens. When the pistons stroke to the top position, the spark plug fires, which ignites the fuel / air mixture, and drives the piston downward. When the piston is in the down position, the exhaust valve opens, directing the exhaust through the exhaust manifold and out through the muffler / tail pipe.

mike690003 · #8 10-19-2003, 05:38 PM

I have checked all vacuum lines today,they are all good and the car still fluctuates at idle. This happened right after I replaced the idle control valve hoses. Everything is hooked up.

Is my idle control valve going bad??

I have cleaned and wd-40'ed it to death.

#9 10-19-2003, 11:53 PM

Quote:

Is that the coolant sensor that controls the aux fan or is it the one that controls the CIS-EZL??

It's the one for the CIS-EZL.

Cheers.

sbourg · #10 10-20-2003, 09:56 AM

"I have checked all vacuum lines today,they are all good and the car still fluctuates at idle. This happened right after I replaced the idle control valve hoses. Everything is hooked up."

In our 190E, when I did this I got the valve in backwards, with similar symptoms. Direction was counter-intuitive (intake below and outlet above), and even when I discovered the error, I don't know why it caused this behavior, but it did.

Steve